The optical calibration process for interferometry based flying height measurements is an important process in determining the accuracy and repeatability of head-disk spacing measurements. The light intensity calibration is typically performed by measuring the maximum and minimum fringe intensity of a light signed while changing the head-disk spacing by at least ¼ wavelength of the light signed. Existing calibration methods use loading and unloading method to alter the spacing of the disk and slider, such as the method disclosed in U.S. Pat. No. 5,457,534, Lacey et al. The intensity Iout of the light reflected from both the slider and from the glass surface of a transparent test disk based on multi-beam interference is given by:
                                          I            out                                I            in                          =                                            r              1              2                        +                          r              2              2                        +                          2              ⁢                              r                1                            ⁢                              r                2                            ⁢              cos              ⁢                                                          ⁢              δ                                            1            +                                          r                1                2                            ⁢                              r                2                2                                      +                          2              ⁢                              r                1                            ⁢                              r                2                            ⁢              cos              ⁢                                                          ⁢              δ                                                          (        1        )            where r1 and r2 are fringe intensities of the light reflected off the slider r1 and the light reflected off the disk r2, and Iin is the incident light intensity. The phase angle difference δ between two reflections is a function of the phase shift φs on reflection off the slider surface. The value of δ can be derived as,
                    δ        =                                            4              ⁢              π              ⁢                                                          ⁢              h                        λ                    +                      2            ⁢                                                  ⁢            π                    -                      ϕ            s                                              (        2        )            where λ is the wavelength of the light, h is the flying height and φs is the phase shift. The intensity of the resultant light detected at the detector changes as the distance h between the disk surface and the slider surface varies from a low (loaded) to high (unloaded) value. FIG. 1 shows a typical plot of intensity changes against time for the load/unloading process.
As the flying height (FH) is reduced to the sub-10 nm region, a higher negative pressure air-bearing is typically required to generate a high suction force which pulls the slider closer to the disk surface and increases the flying stability of the slider. As a result, the suction force, which is contrary to the unloading force, causes the slider to vibrate when the slider is forced to withdraw from the disk. This vibrational behaviour is clearly discernable in the example curves 100, 102, 104 shown in FIG. 1 for different light signal wavelength, in the region between the loaded position 106, and the unloaded position 108. This vibrational behaviour results in a “smearing” of the periodic intensity changes corresponding to the interference fringes, making a determination of the intensity values for the maxima and minima difficult and inaccurate. This can affect the calibration and measurement accuracy.
The unloading position of the slider is another important factor which can affect the measurement accuracy. Due to the pivoting position of the suspension, the unloading process will affect the pitch angle of the slider. An higher pitch angle will increase the fall-off of the maximum and minimum of the intensity and consequently increase the error in the measurement. Another drawback for the load/unload calibration method is, that repeated landing of the slider on the disk may damage and contaminate the air bearing surface (ABS) of the slider. The damages are more significant when an extremely low FH slider is used in the measurement.
Generally, other than the load/unload method, any mechanism which unloads the head from the disk by a quarter wavelengths or more can be used for the maximum and minimum intensity calibration. For example, in the disk drive system, the relative linear speed between the head and recording media is controlled by the rotational speed of the spindle motor. Increasing the spindle speed will increase the separation of the head and media by changing the air-bearing pressure of the slider. By changing the separation between the head and media, the interferometric intensity varies enough to detect at least one maximum and minimum fringe intensity which can be used for the calibration process, as disclosed in T. Ohkubo et al., “Accurate Measurement of Gas-lubricated Slider Bearing Separation Using Visible Laser Interferometry”, Journal of Tribology Transaction of the ASME, October 1987, pp. 1-6. FIG. 2 shows the intensity plot of what is termed the RPM calibration process. Curves 200, 202, and 204 for different light signal wavelength, show that lower slider vibration can be achieved by using the RPM calibration process compared to the load/unload calibration. Consequently, this will reduce the calibration noise and increase the measurement accuracy.
However, in the RPM calibration process the slider ABS design must be such that the FH will increase to above λ/2 by changing the RPM of the spindle. Current ABS slider with extremely low FH, say less than 10 nm, are not able to fly as high as λ/2 even at very high spindle speed.
A need therefore exists to provide an optical flying height calibration and measurement technique that seeks to address at least one of the above mentioned disadvantages.